Coated wire for use in prestressed concrete structures and method of producing same

ABSTRACT

Wire for prestressed concrete structures. The wire is initially produced by cold reduction from a high-carbon material whereby to produce a high-strength wire having a tensile strength of about 250 K s.i. (1,725MN/m.2). The surface of such wire may have a drawing compound embedded in it. This wire is first coated with a chromic acid, phosphoric acid solution which is then dried on the wire. A very thin organic coating, such as an organic phosphate, is then applied to the wire having the chromic acid, phosphoric acid coating dried thereon. The wire, now having the organic coating applied thereto, is then cured.

United States Patent [151 3,640,776 Haney 1 Feb. 8, 1972 s41 COATED WIREFOR usr: IN 3,382,081 5/1968 Cutter m1 ..117/134 x PRESTRESSED CONCRETE2,992,131 7/1961 Bricknell et a1. ..148/6.15 Z X STRUCTURES AND METHODOF PRODUCING SAME [72] Inventor: Eugene E. Haney, Middletown, Ohio [73]Assignee: Armco Steel Corporation, Middletown,

Ohio

[22] Filed: Sept. 10, 1969 [21] App1.No.: 856,836

[52] U.S.Cl ..148/6.l6,117/128,117/134, 148/315 [51] Int. Cl. ..C23t7/26 [58] FieldofSearch ..148/6.16, 6.15; 117/134, 128

[56] References Cited UNITED STATES PATENTS 2,067,214 1/1937 Tanner eta1 ..148/6.16 X

2,191,435 2/1940 Ballard et a1 ..148/6.15 R

2,591,625 4/1952 Simonsson et a1. ..148/6.15

OTHER PUBLlCATlONS Churikov, Chem. Abs. Vol. 57 p. 426g July 1962148-6.]6

Primary Examiner-Ralph S. Kendall Attorney-Melville, Strasser, Foster &Hoffman [57] ABSTRACT Wire for prestressed concrete structures. The wireis initially produced by cold reduction from a high-carbon materialwhereby to produce a high-strength wire having a tensile strength ofabout 250 K s.i. (1,725MN/m. The surface of such wire may have a drawingcompound embedded in it. This wire is first coated with a chromic acid,phosphoric acid solution which is then dried on the wire. A very thinorganic coating, such as an organic phosphate, is then applied to thewire having the chromic acid, phosphoric acid coating dried thereon. Thewire, now having the organic coating applied thereto, is then cured.

7 Claims, No Drawings BACKGROUND OF THE INVENTION 1. Field of theInvention The wire of this invention is especially designed for use ineffecting prestressed concrete structures of either the pretensioned orthe posttensioned type wherein it is imperative to obtain and maintain avery thin coating which will prevent the formation of stress raisers, alocalized cracking condition, in the wire when associated with theconcrete. Since this wire will usually be shipped to the point ofapplication under conditions wherein it will be necessary for the wireto be exposed to the-elements prior to the time it is incorporated withthe concrete structure, it is also necessary that this wire will behighly resistant to general corrosion. Additionally, particularly whenused in pretensioned concrete structures, the coating must be such as toeffect a firm bond of the wire to the concrete. v

By way of additional background, the following'should be notedwithrespect to the terms prestressed, pretensioned" and posttensioned.Prestressing is the art of permanently joining two or more stressedbodies in such a manner that the force'in'one member is balanced by anopposite force in the other member. By such means, compressive energycan be stored up in a brittle or cracked body of sufiicient magnitude tocompensate for any tensile stresses that may be imposed by futureloads.'Generally speaking,-concrete has excellent properties incompression and very poor tensile properties. Steel, on the other hand,has excellent tensile properties. Prestressed concrete combines'thesetwo properties in a single structural member, using the concrete tohandle the compressive stresses and the steel wire or strand to handlethe tensile stresses. This is achieved either by pretensioning orposttensioning.

In pretensioning, long lengths of high-strength stress relieved materialare tensioned by hydraulic or mechanical loading and anchored to endabutments before the concrete is poured. After the concrete has cured,the load is removed from the end abutments and transferred by physicalbond to the concrete member. The high-strength, stress-relievedmaterials are then cut off at the ends of each member, or in some casesthe concrete section may be sawed into lengths by an abrasive cutoffwheel. Pretensioning is particularly adaptable to centrally locatedyards and widely used for mass production of smaller members. In thecured structure the pretensioned wire or strand remains stressed byvirtue of the fact that it is bonded to the concrete substantiallythroughout its length; a proper coating on the wire insures this result.

In posttensioning, which lends itself particularly well to onthe-siteconstruction of larger members, the member is cast with-spaces properlypositioned in the form to allow the ten- -sioning material to passthrough. After the concrete has reached su'flicient compressivestrength, special end anchorages are attached and the tensioning memberstressed. In some instances the remaining space in the concrete memberis filled with grout. The anchorage force may then react to bearingplates cast in the beams or it maybe distributed directly into thesurrounding concrete by other suitable means.

2. Description of the Prior Art Although a search of the prior art hasbeen conducted in order to ascertain the most pertinent of such art, no.assertion is made that the best'art was indeed located. The followingpatents, however, appear to be most representative of the art to whichthis invention relates. U.S. Pat. No. 3,074,827

(Hoover) discloses a process of treating a steel sheet having asubstantially single-phase, alloyed zinc coating containing alu- U.S.Pat. No. 3,094,44l (Curtin) discloses providing an oxida ation-inhibiting and bonding coat onaluminum, zinc and fer- 'rous metalsurfaces by applying thereto solutions'which contain both chromic acidand phosphoric acid. U.S. Pat. No. 3,127,288 (Hines et al.) discloses amethod for treating zinc and zinc-iron alloyed surfaces to protect suchsurfaces against corrosion or discoloration by treating the surfaceswith aqueous solutions containing phosphate ions and chromate ions. U.S.Pat. No. 3,370,992 (Ilenda et al.) discloses a composition and processfor conditioning metal surfaces to receive a sub sequent protectivecoating, the conditioning composition comprising a chromic acid,phosphoric acid bath and the subsequent protective coating beingorganic; the purpose of the procedure is to make the metal surfacereceptive to paint and the like for decorative purposes. German Pat. No.856,545 discloses the treatment of zinc coatings with aqueous solutionsof phosphoric acid and chromic acid.

One of the problems in the past has been that the wire, when shipped tothe construction site, will corrode prior to the time that it is made apart of the concrete structure. Another of the problems has been to findawire which will bond properly with the concrete. The known prior artdoes not provide or suggest a solution for these problems. Both of theseproblems, however, are solved by the instant invention.

In the past the prior art has attempted to meet the problems justdiscussed by not treating the wire at all for it was believed just asundesirable to have a coating on the wire as it was to have rustthereon; it was believed that anything which came between the metal andthe concrete was detrimental, whether this be rust, oil or any othercoating. Accordingly, it was the practice in the prior art to hand wrapthe wire in packages to prevent rusting (vapor phase inhibitors oftenbeing included) and to then rush the packaged wire to the customer,hopefully for immediate use so that rusting would not occur. Theelimination of this expensive packaging and shipping operation has beenone of the chief advantages realized by this invention. An addedadvantage, in pretensioned concrete and metal structures, is that animproved bond is obtained by virtue of the novel coating applied to themetal.

SUMMARY OF THE INVENTION This invention resides in providing a wire forprestressed concrete structure which, even after prolonged exposure tothe elements, will not corrode (other than superficially, which can beeasily wiped off) and which will permit a highly efiicient bond to theconcrete when it is incorporated in pretensioned structures. And,whether used in pretensioned or in posttensioned structure, it will notfail due to stress raisers caused by stress corrosion cracking asdistinguished from general corrosion. The wire is first coated with achromic acid, phosphoric acid solution whereafter a second organiccoating is applied thereto, the coating thus applied on the wire thenbeing cured. Both coatings are necessary. The phosphoric acid acts toremove the drawing compound from the wire and present a wettable surfacefor reaction with the chromic acid. The chromate ions which aredeposited on the surface of the wire provide some corrosion protectionThe extremely thin, perhaps on the nature of 1 molecular thickness,organic coating not only acts to provide the desired bond to concretebut it also cooperates with these chromate ions to make the finishedwire corrosion resistant. In this manner a corrosion resistant wirewhich will efficiently bond to concrete when used in prestressedpretensioned structures is produced.

DESCRIPTION OF THE PREFERRED EMBODIMENT and phosphoric acid in the rangeof 2.0 to 10.0 percent.

Although this is a desirable and workable range for the coatingsolution, it is not critical. It has been determined, for example, thata coating solution which is comprised of 5 percent phosphoric acid and-2percent chromic acid will produce satisfactory results. It is believednecessary, however, to have these acids present in at least the amountindicated; that is, the ranges set forth are to be considered asminimum. Thus, although there may be more than 10 percent phosphoricacid present, there should never be less than 2.0 percent. Similarly,there should always be at least 0.5 percent of the chromic acid,although this may go above 2.0 percent (generally, however, amounts ofchromic acid above 2.0 percent are not used). Specifically, a desirable,workable chromic acid solution comprises about 2 grams of chromiumtrioxide (CrO to about 98 grams of water. A desirable, workablephosphoric acid solution is made up of about 3 to 7 grams of phosphoruspentoxide (P with about 97 to 93 grams of water.

Preferably the phosphoric acid and chromic acid are located in the samecontainer or tank, simply to save time and space. These acids, however,could be located in separate containers. The wire is wetted with theseacids and then promptly dried. The process does not contemplate anactual phosphating or pickling action. The primary function of thephosphoric acid is to react with any impurities which may be found onthe wire and to wet the surface of the wire. In a typical operation, thewire is moved through a 2-foot-long phosphoric acid, chromic acid bathat the rate of 150 feet per minute (0.76 m./sec.). In such an operation,the wire is immediately dried by means of a 360 air wipe, thuseliminating any excess acid on the wire. In effect, a film of chromiumtrioxide is put on the wire and then subjected to this 360 air wipe sothat as thin a film as possible is obtained without wiping all of thischromium trioxide off. Respecting the phosphoric acid, if there shouldbe any of this left on the surface of the wire, it will form an ironphosphate which is not undesirable.

Secondly, an organic coating, extremely thin, preferably on the natureof 1 molecular thickness, is applied to the dry, acidcoated wire. Inpractice it has been found that an organic coating such as succinic acid(one such satisfactory coating material is known in the trade asDacromet) will produce good results when applied according to theteachings of this invention. Other organic coatings may suffice. It isnecessary, however, that these organic coatings of this second treatmentmust be very thin and such that they will be compatible and reactivewith the chromic acid. It is also important that the organic coating bewettable with, and bondable to, concrete. It may be desirable for theorganic coating to have powdered zinc entrained therein.

As indicated, the organic coating should be very thin. Accordingly, assoon after it is applied as possible, it is dried. When the coated wireis used in pretensioned concrete structures there must .be a bond,through the coating, between the wire and the concrete. If the coatingis more than 1 molecule thick, even if the lcoating materials havereacted together, there could be a shearing action within the coatingitself. On the other hand, if the coating is only 1 molecule thick, thenthe effect of this is that one side of such molecular thickness isadhered to the concrete and the other side to the steel, wherebyshearing is presented for the molecule itself cannot be sheared exceptwith atomic forces which are not encountered in these applications.These very thin coatings, therefore, prevent internal organic failure.

After the organic coating has been applied to the previously acid coatedwire and dried, the wire is heated so as to insure a reaction betweenthe organic substance and the chromate. This heating will usually be inthe range of 450 to 800 F. (500 to 700 K.) It is believed that thisreaction heating results in reducing, by the organic compound, thechromium trioxide, which is not reactive with iron, to chromium dioxide,which is reactive with iron, whereby to insure that the coating adheresfirmly to the wire. In this manner it is believed that a chemical bondbetween the coating and the wire is effected.

This third step, therefore, may be considered as a curing operation inwhich the wire, now having the chromic acid, phosphoric acid base coatapplied thereto, and on which coat there is the very thin organiccoating, is heated. A curing range of 450 to 800 F. (500 to 700 K.) hasearlier been indicated, and the time required may vary from 5 seconds to30 minutes. The reaction time is very short but the total time will varywith the nature of the heat source. Actually, however, curing can varyfrom room temperature to 800 F. (700 K.) depending on the organiccompound used and the time available for treatment.

This thin organic coating, presenting a wettable and bondable surfacewith concrete, applied to a previous coating having a chromate therein,gives the results desired. Not only will a wire so coated bond toconcrete when used in pretensioned structures, but also stress corrosioncracking, whether the wire is used in pretensioned or in posttensionedconcrete structures, is prevented. General corrosion is also preventedby the coating resulting from this invention. This insures that stressraisers will not occur and the wires, whether used in pretensioned orposttensioned structures, will be prevented from failing prematurely.

The wire itself is also important. Generally, the wire constituting apart of this invention is produced by cold reduction of a high-carbonmaterial such that the resulting wire has a tensile strength of about250 K s.i. (l,725MN/m.). Before treatment in accordance with theteachings of this invention, the surface of such wire will usually havea drawing compound such as calcium stearate embedded in it. In view ofthese factors, it is necessary that the surface protection provided bythis invention must be of such a nature that, in addition to providingthe corrosion resistant and bonding properties heretofore mentioned, itwill not produce stress raisers in the wire surface nor will it annealthe cold-drawn structure. The coatings herein described, actingtogether, achieve all of these results. Any hydrogen generated anywherealong the line must be kept away from the steel because it would diffuseinto the steel in the stressed condition and cause cracking. The hereindescribed coating forms a film which prevents hydrogen diffusion.

The cold-drawn wire of this invention must also be stress relieved. Thisis easily accomplished by using a molten lead pot in the range of 750 to800 F. (670 to 700 K.), as is well known in the art. The placing of thisstress reliever is not critical and could even occur as a part of thereaction heau'ng step needed for the organic substance and the chromate.For example, an induction heating step could be employed to effect botha stress relief of the cold-drawn wire and the reaction between thechromate and, organic substance; thus the earlier suggested range of 450to 800 F. (500 to 700 K.) can be used.

Wires have been referred to throughout this description. There may be,however, not only individual wires but also strands made up of aplurality of wires. Common individual wire sizes are 0.192 inch, 0.250inch and 0.276 inch (respectively 50, 65, and 70 mm.) in diameter. Themost commonly used strand sizes are one-fourth inch, five-sixteenthsinch, three-eighths inch, seven-sixteenths inch, one-half inch, and0.600 inch (respectively 65, 80, 95, 110, 130, and 150 mm.) diameterseven-wire strand. Such wires, or wires made into strands, forprestressed concrete are preferably made from heat-treated, high-carbonrods with multiple-holecold drafting to produce the required mechanicalproperties. It must have a high tensile strength (usually on the orderof 240,000 (1 ,650MN/m. pounds per square inch minimum); it must havehigh yield strength (at least percent of tensile); it

must have adequate ductility for placement, anchorage and tensioning;and it must have uniform stress-strain characteristics within theelastic range. The prestressing steel must also be relatively straightto facilitate its placement in concrete, and free from excessive amountsof foreign material on its surface which might adversely affect itsbonding characteristics. Individual wires and strand arepresently stressrelieved by beating them in molten lead. Strands are particularly wellsuited for pretensioned structures in that the presence of v the valleysmakes for additional mechanical bonding between the strand and theconcrete.

It is believed that the phosphoric acid acts to react with the drawingcompound from the wire and present a wettable surface for reaction withthe chromic acid. The chromate ions which are deposited on the surfaceof the wire themselves provide some corrosion protection while at thesame time acting with the organic coating to not only secure theefficient bond desired but also to increase the corrosion-resistantproperties in the finished wire of this invention.

The wire, after the organic coating treatment, looks somewhat like alightly pickled surface with no coating on it. In this connection, it isemphasized that the total coating thickness must be very thin so that ashear failure is prevented within the coating layer itself. This is tobe distinguished from the prior art wherein an inferior pickling orcleaning preparation, along with an ordinary paint coating, will notachieve the results produced by this invention. Previous to thisinvention, the wires were uncoated and the resulting corrosion productwhich occurred seriously detracted from the bond strength which could bedeveloped in concrete.

The wire of this invention, as produced by the method described herein,is highly resistant to corrosion and it adheres nicely in a bondingaction with the concrete of the prestressed and pretensioned concretestructures with which it is used.

It is believed that the foregoing constitutes a full and completedisclosure of this invention. Numerous variations and modifications maybe made without departing from the scope and spirit of this invention,and no limitations are intended except as specifically set forth in theclaims which follow. It has been determined, for example, that otherpossible organic coatings, identified by brand name, are Dacromet 2(believed to be basically succinic acid with suspended zinc particles)and Rustarest, an organic phosphate.

It is especially to be noted that in the claims which follow the use ofthe word wire is intended to include strand as earlier described in thisspecification. The claims, therefore, are to be considered as directedto and embracing both a single wire and a multitude of wires woundtogether to comprise a strand.

The embodiments of the invention in which an exclusive property ofprivilege is claimed are defined as follows:

1. A wire for prestressed concrete structures comprising a coated steelwire having a high tensile strength, the coating being the end reactionproduct of an aqueous solution of phosphoric acid in which thephosphoric acid is present in at least the range 2.0 to 10.0 percent,and an aqueous solution of chromic acid in which the chromic acid ispresent in at least the range of 0.5 to 2.0 percent, and an organiccompound wettable with and bondable to concrete and compatible andreactive with the chromic acid on said wire, said coating being verythin, on the order of 1 molecule thick.

2. A wire for prestressed concrete structures comprising a thinly coatedsteel wire having a high tensile strength, said coating being the endreaction product of P and water in which the P 0 is present in at leastthe range of 2.0 to 10.0 percent, CrO and water in which the C10 ispresent in at least the range of 0.5 to 2.0 percent, and an organiccompound wettable with and bondable to concrete and compatible with andreducing to the CrO whereby to form CrO said coating being very thin, onthe order of 1 molecule thick.

3. A method of making wire for prestressed concrete structures whichcomprises providing a steel wire having a high tensile strength, passingsaid steel wire briefly through aqueous solutions of phosphoric acid inwhich the phosphoric acid is present in the range of at least 2.0 to10,0 percent and chromic acid in which the chromic acid is present inthe range of at least 0.5 to 2.0 percent, promptly drying said steelwire, passing said steel wire through an organic compound wettable withand bondable to concrete and reducing to and compatible with chromicacid (CrO drying said organic compound, and heating said steel wire toeffect a reaction between said chromic acid on said wire and saidorganic compound, whereby to achieve a final coating on said wire whichis on the order of 1 molecule thick.

4. The method of claim 3 including the step of stress relieving thesteel wire.

5. A wire for prestressed concrete structures, said wire having anundercoating thereon comprised of the reaction products of an aqueousphosphoric acid, chromic acid solution applied thereto, said phosphoricacid solution having phosphoric acid in the range of at least 2.0 to10.0 percent and said chromic acid solution having chromic acid in therange of at least 0.5 to 2.0 percent, and a very thin outercoating onsaid undercoating, said outer coating resulting from the application ofa thin layer of a liquid organic coating wettable with and bondable toconcrete and compatible and reactive with the chromic acid on the wire,whereby said wire is resistant to corrosion and to stress corrosioncracking and whereby said wire bonds to concrete with high efficiency.

5. The wire of claim 5 in which the total thickness of the under andouter coatings is very thin, said outer coating being on the order of 1molecular thickness, whereby to prevent shear failure within thecoatings.

7. A method of producing a wire for pretensioned and posttensionedconcrete, which wire is resistant to corrosion and to stress corrosioncracking and which wire has a highly eificient bond to concrete,comprising the steps of applying an aqueous solution of phosphoric acid,chromic acid to the wire, said phosphoric acid solution havingphosphoric acid in the range of at least 2.0 to 10.0 percent and saidchromic acid solution having chromic acid in the range of at least 0.5to 2.0 percent, drying the solution on the wire, applying a thin organiccoating to the dry acid-coated wire, said organic coating being wettablewith and bondable to concrete and compatible and reactive with chromicacid, and curing the thus-coated wire to dry said organic coating.

2. A wire for prestressed concrete structures comprising a thinly coatedsteel wire having a high tensile strength, said coating being the endreaction product of P2O5 and water in which the P2O5 is present in atleast the range of 2.0 to 10.0 percent, CrO3 and water in which the CrO3is present in at least the range of 0.5 to 2.0 percent, and an organiccompound wettable with and bondable to concrete and compatible with andreducing to the CrO3 whereby to form CrO2, said coating being very thin,on the order of 1 molecule thick.
 3. A method of making wire forprestressed concrete structures which comprises providing a steel wirehaving a high tensile strength, passing said steel wire briefly throughaqueous solutions of phosphoric acid in which the phosphoric acid ispresent in the range of at least 2.0 to 10.0 percent and chromic acid inwhich the chromic acid is present in the range of at least 0.5 to 2.0percent, promptly drying said steel wire, passing said steel wirethrough an organic compound wettable with and bondable to concrete andreducing to and compatible with chromic acid (CrO3), drying said organiccompound, and heating said steel wire to effect a reaction between saidchromic acid on said wire and said organic compound, whereby to achievea final coating on said wire which is on the order of 1 molecule thick.4. The method of claim 3 including the step of stress relieving thesteel wire.
 5. A wire for prestressed concrete structures, said wirehaving an undercoating thereon comprised of the reaction products of anaqueous phosphoric acid, chromic acid solution applied thereto, saidphosphoric acid solution having phosphoric acid in the range of at least2.0 to 10.0 percent and said chromic acid solution having chromic acidin the range of at least 0.5 to 2.0 percent, and a very thinoutercoating on said undercoating, said outer coating resulting from theapplication of a thin layer of a liquid organic coating wettable withand bondable to concrete and compatible and reactive with the chromicacid on the wire, whereby said wire is resistant to corrosion and tostress corrosion cracking and whereby said wire bonds to concrete withhigh efficiency.
 6. The wire of claim 5 in which the total thickness ofthe under and outer coatings is very thin, said outer coating being onthe order of 1 molecular thickness, whereby to prevent shear failurewithin the coatings.
 7. A method of producing a wire for pretensionedand posttensioned concrete, which wire is resistant to corrosion and tostress corrosion cracking and which wire has a highly efficient bond toconcrete, comprising the steps of applying an aqueous solution ofphosphoric acid, chromic acid to the wire, said phosphoric acid solutionhaving phosphoric acid in the range of at least 2.0 to 10.0 percent andsaid chromic acid solution having chromic acid in the range of at least0.5 to 2.0 percent, drying the solution on the wire, applying a thinorganic coating to the dry acid-coated wire, said organic coating beingwettable with and bondable to concrete and compatible and reactive withchromic acid, and curing the thus-coated wire to dry said organiccoating.